Electrical stimulation of neural tissue is used for several purposes, including pain control, diagnosis and neural rehabilitation. For example, a cochlear implant (CI) is a small electronic device, part of which is placed under the skin, behind the ear, of a profoundly deaf or severely hard of hearing person. The cochlear implant receives signals from a microphone and electrically stimulates auditory nerves of the wearer. Although hearing through a CI may be different than normal hearing, the wearer perceives sounds and another person can orally communicate with the wearer.
CI and other neural stimulation is accomplished by placing at least one electrode near neural tissue and sending an electrical signal to the electrode. The electrical signal is produced with reference to a second (ground) electrode. That is, the signal is delivered across the two electrodes. The second electrode may be placed near the neural tissue or some distance from the neural tissue. The neural tissue is typically stimulated with a bi-phasic pulse, i.e., first a positive pulse is sent to the electrode, followed almost immediately thereafter by a negative pulse.
Better results are obtained from better focused electrical stimulation. Neural tissues are typically very small. To achieve desired results, selected individual nerves should be stimulated. However, prior art neural stimulation apparatus is unable to deliver well-focused electrical stimulation to neural tissue.
A conventional longitudinal bipolar neural stimulation scheme includes two conducting electrodes arranged side-by-side on an insulated carrier. A signal is fed to a first of the two electrodes, and the other electrode acts as a ground. During the positive phase of the stimulation signal, current flows from one of the electrodes, through the stimulated tissue, into the other (ground) electrode. During the negative phase of the stimulation signal, the current flows in the opposite direction, i.e., out of the ground electrode, through the stimulated tissue, and into the first electrode. Consequently, each of the electrodes acts as a stimulation point, and the stimulation is not well focused.
A conventional longitudinal tripolar scheme includes two ground electrodes flanking a center electrode. In this case, the amount of current that flows through each of the two flanking electrodes is one-half the amount of current that flows through the center electrode. Tripolar schemes produce reasonably well focused stimulation, however they require three electrodes.